Simple wrapper over F/J CountedCompleter to support priority
queues. F/J queues are simple unordered (and extremely light weight)
queues. However, we frequently need priorities to avoid deadlock and to
promote efficient throughput (e.g. failure to respond quickly to TaskGetKey can block an entire node for lack of some small piece of
data). So each attempt to do lower-priority F/J work starts with an
attempt to work and drain the higher-priority queues.

If this is a F/J thread, return it's priority - used to lift the
priority of a blocking remote call, so the remote node runs it at a
higher priority - so we don't deadlock when we burn the local
thread.

currThrPriority

protected static byte currThrPriority()

If this is a F/J thread, return it's priority - used to lift the
priority of a blocking remote call, so the remote node runs it at a
higher priority - so we don't deadlock when we burn the local
thread.